1. Field of the Invention
The present invention relates to solid lubricants and methods for forming self-lubricating coatings. More particularly, the present invention relates to self-lubricating coatings which reduce the friction and wear of components or parts, such as piston rings, cylinder liners, gears, heavy duty rear axle spiders, and rollers, especially, parts working under difficult lubrication conditions.
2. Brief Description of the Related Art
It is well known that surface characteristics play an important role in friction and wear processes, and that the most important reason for using lubricants is to reduce friction. Oils are commonly used lubricants, coating surfaces to reduce friction, thereby allowing surfaces to slide more easily and reducing wear. However, the number of machines and mechanisms working in extreme conditions increases every year, and oil is not generally suitable for use under these conditions. To address the problems associated with movable joints in a vacuum, in low and high temperatures, and for the successful operation of tribotechnical units in these conditions, it is necessary to develop alternative lubrication materials and methods.
Alternatives which have been developed for use at elevated temperatures include solid lubricants such as molybdenum disulfide and graphite. Various self-lubricating, solid, composite coating materials have been developed at NASA. In one; (PS200), chromium carbide serves as a tough, wear-resistant matrix, while silver particles provide lubrication at low temperatures and a barium fluoride/calcium fluoride eutectic phase provides lubrication at higher temperatures.
Sulfur is an important component in many organic and inorganic lubricants, providing unique lubricity and passivating characteristics against mechanical or chemical attack on surfaces. Some studies have shown that a monolayer of strongly chemisorbed atoms, such as may be found in sulfur, can act as a lubricant, significantly decreasing the friction coefficient of the surface. Without being bound by theory, the good lubricating properties of sulfur compounds may therefore derive from the strong sulfur-metal bonds formed at the surface. During mechanical contact, sulfur overlayers can thus prevent the formation of metal-metal bonds between the contacting bodies. Therefore, for thin solid lubricant films, the friction coefficient can be remarkably stable, despite the fact that much of the sulfide coating is worn early in the life of the contact. Because of its low melting point, and because of its layer lattice (hexagonal crystal structure) crystal structures, sulfur furthermore has good lubricating properties especially in a vacuum.
A low temperature sulfurizing method is currently used in industries in, e.g., Japan, France, India and China. This method (which is called Sulf-BT process or Caubet process, available under license from HEF France) is an anodic sulfurization performed in a suitable molten bath. It forms a thin (a few microns thick) pyrrothite (Fe1xe2x88x92xS, a metal-deficient iron sulfide) film on steel. The method can be used for various irons and steels being characterized as low-temperature, rapid, and with no dangers of hydrogen embrittlement.
However, almost all of the bath compositions used for Sulf-BT process contain at least one harmful substance, such as NaCNS, KNCS, and KCN. The Sulf-BT process can furthermore generates harmful gases or liquids, even at operation temperatures of below 200xc2x0 C. After sulfurizing, the parts or specimens have to be rinsed in running water to dissolve the frozen salt crust, potentially producing a hazardous waste stream, ands sulfurized parts or samples are easily corroded by retained salt if rinsing is not thorough. The molten salt loses efficacy after few cycles, and the waste salt, which may also be hazardous, is difficult to regenerate efficiently. Finally, the Sulf-BT process is not suitable for steel having a chromium content higher than 12%, or for other, nonferrous metals.
Molybdenum disulfide (MoS2) may be applied to surfaces as a solid lubricant by a number of methods, including simple rubbing or burnishing, air spraying of resin-bonded or inorganically bonded coating, and more recently, physical vapor deposition (PVD) techniques as sputtering. Burnished films are the easiest and least expensive to apply, but have very limited wear life. Resin-bonded spray coatings, especially the heat-cured variety, have good wear life and are frequently used in ordinary air environments. They are typically 5 to 15 microns thick. Their coefficient of friction depends on humidity and sliding conditions, as well as the binder material used.
The friction coefficients for sputtered MoS2 is about 0.01 to 0.15. However, the sputtered films are very thin, usually 0.1 microns to 1.5 microns, because thicker films require longer sputtering times. The sputtering apparatus is generally very expensive and the cost of production is high. Some workers have shown that the durability of the MoS2 films depended largely on the type of sputtering apparatus used.
Formation of solid lubricant coatings by thermal spray processes has been disclosed in U.S. Pat. Nos. 5,763,106, 5,332,422 and 5,484,662. U.S. Pat. No. 5,763,106 discloses thermal spray of composite powder containing a ceramic, metal and lubricant. In U.S. Pat. Nos. 5,332,422 and 5,484,662, the powders for thermal spray comprise agglomerates of two more solid lubricant particles, together with fusible metal particles such as steel, bound together with a binder such as wax. The lubricant particles may further be coated with a metal such as copper.
Pure sulfide coatings (i.e., comprising only lubricant) have heretofore not been available by thermal spray, as sulfide self-lubricants are thermally unstable. The extremely high temperature zone of plasma gun would thus be expected to result in decomposition and oxidation of the sulfide-based lubricant. A need exists, therefore, for materials and methods for the production of sulphide-based lubricating coatings by thermal spray methods.
The above-described drawbacks and disadvantages are solved or alleviated by the present process, comprising thermal spray of agglomerates of sulfur-coated lubricant particles. The sulfur coating acts as a binder for the particles, and prevents thermal decomposition of the lubricant particles during the spraying process. The advantages of using thermal spray techniques are that the process is environmentally benign, low cost, east to operate, and leaves no corrosion from residual salts. Thermal spray also offers versatility, e.g., large parts are easy to spray, and a wide variety of substrates may be sprayed. Thermal spray methods are also readily adaptable to continuous manufacturing processes. In another advantageous feature, the lubricants may comprise nanostructured materials, which after thermal spray results in coatings having nanostructured coatings.